Try to listen to samples of music before you download full songs. If you're paying for songs individually, don't just download blindly. Make sure the file you are downloading is the right song and sounds high quality. Always take a few seconds to listen to the sample before spending your money.
When looking for a website to download music, look for one that offers discounts. Many music download sites offer discounts Witch which are specific to the type of music you listen to. To find these types of promotions, sign up to receive the website's promotional newsletter or search the name of the website and coupons.
If you download a ton of music, it may be better to use a subscription service. Rdio or Spotify give you unlimited access for a monthly fee. You can then download music you like to hear offline just like if you were to pay for the download. It might be a more inexpensive way to expand your music library.
If you want to subscribe to Google Play, you will need to use a Spotify desktop app to access the music. At the moment, this is the only way to be able to use Google Play off of its website. That said, new apps are in the process of being created right now.
Be sure to check the file format before downloading any music. You've got to be sure that the file you get is compatible with whatever system you are using. If unsure, a good bet is to stick with MP3 files. This file extension is about as universal as they come.